In a scanning electron microscope or a scanning transmission electron microscope that scans a specimen with an electron beam to obtain a magnified image of the specimen, various kinds of electrons are obtained due to the interaction between the specimen and the irradiation primary electron beam, such as secondary electrons, backscattered electrons (specimen backscattered electrons), specimen forward scattered electrons, and transmission electrons. Depending on the relationship between the specimen and the detection position, the various kinds of electrons are discriminated when detected, and characteristic image contrasts are obtained by respective detectors, whereby a scanning magnified image of the specimen can be formed.
With reference to the scanning transmission electron microscope as an example, three-dimensional specimen information obtained from the specimen image and resolution will be described.
The scanning transmission electron microscope forms an image based on secondary electrons, forward scattered electrons, or transmission electrons that are produced from the specimen when scanned with a minute spot of an electron beam, and has a subnanometer-size attainable resolution.
(i−1) Image Resolution of Forward Scattered Electron Image and Transmission Electron Image.
With the specimen forward scattered electrons, when the scattering angle acquisition angle is large, a contrast referred to as “Z-contrast”, which is proportional to the square of the atomic number Z of the specimen, can be obtained. This image is due to Rutherford scattering by the atomic nucleus, and is indicated by the convolution of the size of the incident electron beam and the size of the specimen atoms. Thus, image resolution is determined in accordance with the spot size of the incident electron.
Transmission electrons consist of electrons that have passed through the specimen and electrons that have lost energy slightly due to inelastic scattering, and form an image similar to an image by the so-called transmission electron microscope. The transmission electron image is separated into amplitude contrast and phase contrast, and an image of atomic resolution is formed by the phase contrast formed by an interference electron beam from the specimen. In the image of atomic resolution by transmission electrons, it is not easy to identify the position of atoms because the contrast formed by the electron beam focused on the specimen is varied. Normally, interpretation based on a comparison with a simulation image is required.
(i-2) Three-Dimensional Information of Forward Scattered Electron Image and Transmission Electron Image
Image formation by specimen forward scattered electrons and transmission electrons provide only planar information. When the specimen has a thickness in the depth direction, because the signal image is due to electrons that have passed through the specimen or to forward scattered electrons, structures or portions of different compositions that exist in the depth direction with respect to the electron beam incident direction are superposed. As a result, there has been the problem that unwanted signals are occasionally observed.
The above characteristics will be described with reference to a specific example. When catalyst particles with a diameter on the order of several to several tens of nanometers are to be observed and analyzed by using the scanning transmission electron microscope, it is necessary to find a field of view in which the support material for the catalyst particles for observation or other catalyst particles do not exist on the axis of observation in a superposed manner. However, in the case of the electron microscope only provided with a forward scattered electron or transmission electron detector, as described above, the observed image is an image that has substantially completely transmitted through the specimen. As a result, the depth direction position cannot be identified, and the field of view is difficult to find. Further, in the image formed by forward scattered or transmission electrons, the image information is two-dimensional and planar, which is not suitable for stereoscopic structure observation in an atomic size.
With reference to FIG. 2, the difference in focal depth between a secondary electron image and a specimen forward scattered electron image will be described. FIG. 2 schematically shows the different ways in which an observed two-dimensional image is viewed. Suppose a state such that specimen structures are perpendicularly arranged with respect to the incident primary electron beam. Then, in a secondary electron image (SE image) and a specimen forward scattered electron image (DF-STEM image), images reflecting the specimen structure are observed as shown. When the specimen structure is not more than several nm or like an atomic arrangement, giving a focusing shift of the irradiation electron beam (defocus Δf) of approximately 20 nm almost eliminates the contrast reflecting the structures in the case of the secondary electron image, while enabling observation with a strong contrast maintained in the case of the specimen forward scattered electron image.
Namely, in the case of image formation by transmission electrons or specimen forward scattered electrons, signals from structures that exist in the specimen depth direction are all projected, so that the stereoscopic structure of the specimen cannot be grasped, nor can the internal structures of the specimen be clearly separated.
(ii-1) Image Resolution of Secondary Electron Image
Image resolution in a secondary electron image is a composition of the two events of electron beam probe size and electron diffusion in specimen.
The probe size of the electron beam is given by expression 1, or the so-called Everhart's formula:
                    [                  Expression          ⁢                                          ⁢          1                ]                                                            d        =                                                            (                                                      1                    4                                    ⁢                                      C                    s                                    ⁢                                      α                    3                                                  )                            2                        +                                          (                                                      1                    2                                    ⁢                                                            C                      c                                        ⁡                                          (                                                                        δ                          ⁢                                                                                                          ⁢                          E                                                E                                            )                                                        ⁢                  α                                )                            2                        +                                          (                                                      0.61                    ⁢                    λ                                    α                                )                            2                        +                                          4                ⁢                                  i                  p                                                                              βπ                  2                                ⁢                                  α                  2                                                                                        Expression        ⁢                                  ⁢        1            
The probe size of the primary electron beam according to expression 1 is given by the electron microscope device factors, and ideally provides the image resolution of the scanning electron microscope. The first term is the influence of spherical aberration (Cs) of the electron lens and proportional to the cube of the electron beam illumination angle α. The second term is the effect of chromatic aberration (Cc) of the electron lens, and dependent on the amount of minute displacement δE of the acceleration voltage E of the primary electron beam and the illumination angle α. The third term is the diffraction aberration. The fourth term is dependent on the function of brightness β of the electron source and electron beam probe current ip.
FIG. 8 shows the relationship between the electron beam illumination angle and the electron beam probe size, showing the relationship between the electron beam probe size and the illumination angle according to expression 1. In a charged particle instrument according to the present invention, when an aberration corrector for correcting the spherical aberration Cs of the electron beam probe is installed, the first term of expression 1 can be made substantially zero. By using an electron microscope of the scanning electron microscope equipped with the spherical aberration corrector, an electron beam probe of not more than 0.1 nm can be formed. The electron beam probe size becomes smaller as the illumination angle is increased according to the terms following the second term. The minimum value is given by the illumination angle α0, from which the probe size becomes greater as the illumination angle is increased. Namely, the image resolution of the obtained scanning magnified image is increased, enabling the observation of smaller structures.
Next, the electron diffusion in specimen will be described. For example, when a secondary electron image of a specimen is to be obtained by using a general-purpose scanning electron microscope with acceleration voltage set lower than 30 kV, the primary electron beam causes multiple scattering in the specimen and has a teardrop-shaped divergence. Thus, secondary electrons are produced from the wide area, resulting in a decrease in image resolution.
Accordingly, while the image resolution of the secondary electron image is due to the composition of the two events of electron beam probe size and electron diffusion in specimen, atomic resolution has not been achieved in conventional secondary electron images.
(ii-2) Three-Dimensional Information of Secondary Electron Image
As a technique for observing a three-dimensional surface structure, namely a topographic observation technique, an observation technique based on the use of secondary electrons has been conventional used. In the conventional secondary electron image, the secondary electrons are emitted from the depth on the order of 2 to 10 nm from the specimen surface layer, depending on the material. The emitted electrons characteristically provide a contrast dependent on the specimen surface morphology because of the effect of increased signal intensity from specimen edge portions according to a cosine angular distribution similar to Lambert's cosine law, for example. Thus, the secondary electron image strongly reflects the three-dimensional information of the specimen.